1. Field of the Invention
The present invention relates to an airbag apparatus for a driver's seat mountable on a steering wheel of a vehicle. More particularly, the invention relates to an airbag apparatus for a driver' seat in which a vent hole of an airbag is covered by a cover portion, which cover portion is continuous with a tether that controls a clearance between a vehicle body side wall and a driver side wall of the airbag at inflation, and the vent hole is opened when the cover portion loosens along with loosening of the tether when the airbag cushions the driver.
2. Description of Related Art
JP 2012-061982 A discloses a known airbag apparatus for a driver's seat with a vent mechanism for preventing undue elevation of airbag pressure, in which a vent hole of an airbag is opened with the aid of loosening of a tether. In the airbag apparatus, the airbag is so inflatable as to separate a vehicle body side wall deployable toward a steering wheel and a driver side wall deployable toward the driver, and includes a tether that connects the vehicle body side wall and the driver side wall for controlling a clearance between the vehicle body side wall and the driver side wall at airbag inflation. The vent hole is formed on the vehicle body side wall and is covered by a cover portion which is formed continuously with the tether. In operation of the vent mechanism, the cover portion loosens and opens the vent hole when the driver side wall cushions the driver and moves toward the vehicle body side wall.
More specifically, the above-described vent mechanism further includes an inner panel that is located on an inner surface of the vehicle body side wall at the location of the vent hole. The cover portion, which is continuous with the tether, is located between the vent hole and the inner panel. The first end of the tether on the side of the cover portion is joined to the vehicle body side wall whereas the second end of the tether away from the cover portion is joined to the driver side wall. The inner panel covers an inner surface of the cover portion and is joined to the vehicle body side wall by its opposite edges in a width direction of the cover portion. The inner panel includes at the center an inner opening so as to allow an internal pressure of the airbag to act on the cover portion.
With this vent mechanism, when the driver side wall cushions the driver at airbag inflation and moves toward the vehicle body side wall, the tether loosens, and the cover portion receives an internal pressure of the airbag through the inner opening of the inner panel and loosens outwardly in such a manner as to pass through the vent hole, and then slips out of a space between the inner opening and the vent hole, thereby opening the vent hole. Thus an inflation gas is released from the vent hole.
However, in the conventional airbag apparatus, when the cover portion loosens and opens the vent hole, a length of a portion of the cover portion passing through the vent hole equals to a loosening amount of only one tether which extends from a peripheral region of the cover portion not joined to the vehicle body side wall.
In other words, the height of the cover portion which has slipped out of the vent hole is only half of a distance that the driver side wall moves toward the vehicle body side wall. This is not enough to open the vent hole sufficiently and release an inflation gas quickly.